linux-kernel - Re: warning: objtool: fn1 uses BP as a scratch register

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Message-ID: <20190719183125.2tuhcch2rtanxvyn@treble>
Date:   Fri, 19 Jul 2019 13:31:25 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: warning: objtool: fn1 uses BP as a scratch register

On Fri, Jul 19, 2019 at 11:23:16AM -0700, Nick Desaulniers wrote:
> On Fri, Jul 19, 2019 at 11:10 AM Arnd Bergmann <arnd@...db.de> wrote:
> >
> > A lot of objtool fixes showed up in linux-next, so I looked at some
> > remaining ones.
> > This one comes a lot up in some configurations
> >
> > https://godbolt.org/z/ZZLVD-
> >
> > struct ov7670_win_size {
> >   int width;
> >   int height;
> > };
> > struct ov7670_devtype {
> >   struct ov7670_win_size *win_sizes;
> >   unsigned n_win_sizes;
> > };
> > struct ov7670_info {
> >   int min_width;
> >   int min_height;
> >   struct ov7670_devtype devtype;
> > } a;
> > int b;
> > int fn1() {
> >   struct ov7670_info c = a;
> >   int i = 0;
> >   for (; i < c.devtype.n_win_sizes; i++) {
> >     struct ov7670_win_size d = c.devtype.win_sizes[i];
> >     if (c.min_width && d.width < d.height < c.min_height)
> >       if (b)
> >         return 0;
> >   }
> >   return 2;
> > }
> >
> > $ clang-8 -O2 -fno-omit-frame-pointer -fno-strict-overflow -c ov7670.i
> > $ objtool check  --no-unreachable --uaccess ov7670.o
> > ov7670.o: warning: objtool: fn1 uses BP as a scratch register
> 
> Thanks for the report and reduced test case.  From the godbolt link, I
> don't see %rbp, %ebp, %bp, or %bpl being referenced (other that %rbp
> in the typical epilogue).  Am I missing something? Is objtool maybe
> not reporting the precise function at fault?

I haven't looked, but it could very well be an objtool bug (surprise).

-- 
Josh